The present invention generally relates to imaging systems for the detection and recognition of remote objects. More particularly, the invention relates to improving polarization data acquisition using polarimetric sensors.
Infrared radiation is often used to detect objects where visible light is either blocked or not present. Thus, it is possible to use infrared detection at night or through clouds, dust or haze. An infrared detector can be in the form of either a single detecting element or an array of such elements to produce an image. The optical receiver can be a photoelectric detector such as an amorphous silicon photodetector or a photodiode that converts the optical signal into an electrical signal that is conducted to an input pin on a receiving IC. A portion of the light reflected off of the surface of an object is detected by a photodetector array.
A limitation when using thermal intensity for search and identification is that thermal intensity gives only one parameter whereas the surface orientation on a three-dimensional object is specified by two angles. Information about the two angles of surface orientation is often contained in the polarization of the thermal radiation (i.e., infrared). Polarization of the thermal radiation also gives useful information about the surface properties of the object. Man-made objects have unnaturally smooth surfaces, which results in radiation with greater polarization. Natural backgrounds such as grass, trees, dirt, and sand generate radiation that is less polarized.
The polarization properties of a beam of incoherent radiation emitted or reflected from a object""s surface can be completely described at a given wavelength by the four Stokes parameters, (I, Q, U and V). The first Stokes parameter I is a measure of the total intensity of radiation. The second parameter Q measures the amount of linear polarization in the horizontal direction. The third parameter U measures the amount of linear polarization at 45 degrees from the horizontal. The fourth parameter V is associated with the circular polarization. Photodetector sensors are used today to capture polarization data. However to improve the accuracy of the object detected additional polarization data is needed. Developing a custom sensor array with additional detector elements is one approach to obtaining more data; however this approach has resulted in an increase in expense and complexity of the sensor array used to capture the polarization data.
Wire grid polarizers have been used in the past to gather polarization data. Wire grid polarizers utilize parallel lines of conductive materials of 5xcexc (micron) scale width spacing deposited onto an infrared transparent substrate. The light having polarization parallel to the conducting lines is absorbed and/or reflected, while the light having polarization perpendicular to the lines is transmitted. Wire grid polarizers have some advantage for operation in the 3.5 xcexcm to 14.5 xcexcm wavelength range since they are more compact and there is no beam offset or angular displacement at normal incidence. However, wire grid polarizers are expensive, extremely delicate and are easily damaged in handling.
Another challenge involves developing thermal sensing systems that utilize commercial off the shelf (COTS) parts versus custom designed parts since many manufacturers prefer to manufacture high volume parts and charge a premium to provide the low volume, custom parts. The use of COTS parts in these systems would not only reduce cost and manufacturing cycle time but would also simplify field repair of such thermal sensing systems.
A system and an arrangement that addresses the aforementioned problems, as well as other related problems, are therefore desirable.
The present invention is directed to addressing the above and other needs in connection with electro-optic sensors used in target acquisition systems. The present approach facilitates the extraction of polarization data to be used in polarization algorithms for identifying and classifying targets.
According to one aspect of the invention, an imaging sensor for polarizing light includes an array of light detecting elements that converts light into a plurality of photocurrent signals. The sensor also includes a rotatable disk positioned in between a light source and the light detecting array and parallel to the light detecting array. The rotatable disk includes a plurality of linear members that are opaque and parallel to each other that polarize light from the light source passed to the light-detecting array. The sensor further includes a circuit arrangement configured to generate a data set of polarization vector components, the polarization vector components being generated as a function of a set of the photocurrent signals that are sampled as a function of a position of the rotating disk.
According another aspect of the invention, a polarimetric infrared imaging sensor arrangement simplifies generating polarization vector data from light reflected from an object, wherein the vector data is used for object identification. The sensor arrangement includes a photodetector array having an optically transparent surface that is arranged to convert any detected light reflected from the object into a plurality of photocurrent signals. The sensor arrangement also includes a rotatable disk positioned between the object and the photodetector array and parallel to the photodetector array, the rotatable disk including a plurality of linear members that are opaque and parallel to each other, whereby the reflected light is polarized and passed to the photodetector array. A rotator arrangement is engaged with a portion of the rotatable disk and arranged to impart a selected rotation rate to the rotatable disk. A circuit arrangement is coupled to the photodetector array and is configured to generate a set of polarization vector data by sampling the photocurrent signals.
According to yet another aspect of the invention, a method facilitates identifying an object in a target classification system by using light reflected from the object as data. In particular, the method includes converting light detected from the object into a plurality of photocurrent signals using a photodetector array. A disk, positioned in between a light source and the photodetector array and parallel to the array, is then rotated to polarize light from the light source before being detected by the photodetector array. The rotatable disk includes a plurality of linear members that are opaque and are parallel to each other. The plurality of photocurrent signals are then sampled at the photodetector array at selected disk angles as the disk rotates. A data set of polarization vector components is then generated from the sampled plurality of photocurrent signals.
It will be appreciated that various other embodiments are set forth in the Detailed Description and Claims that follow.